Tubular endoprostheses such as medical stents are placed within the body to perform a function such as maintaining a body lumen open, for example, a passageway occluded by a tumor or a blood vessel restricted by plaque. Tubular endoprostheses in the form of grafts are used to substitute for or reinforce a lumen, such as the aorta or other blood vessels which have been weakened, e.g., by an aneurysm.
Typically, these endoprostheses are delivered inside the body by a catheter that supports the device in a compacted or otherwise reduced-size form as it is transported to the desired site. The size is particularly small when a percutaneous insertion technique is employed. Upon reaching the site, the endoprosthesis is expanded so that it engages the walls of the lumen.
The expansion mechanism may involve forcing the endoprosthesis to expand radially outwardly, for example, by inflation of a balloon carried by the catheter, to plastically deform and fix the device at a predetermined expanded position in contact with the lumen wall. The expanding means, the balloon, can then be deflated and the catheter removed.
In another technique, the endoprosthesis is formed of a highly elastic material that can be reversibly compacted and expanded. During introduction into the body, the endoprosthesis is restrained in the compacted condition and upon reaching the desired site for implantation, the restraint is removed, enabling the device to self-expand by its own internal elastic restoring force.
In many cases, X-ray fluoroscopy is used to view an endoprosthesis within the body cavity to monitor placement and operation. The device may also be viewed by X-ray film after placement for medical follow-up evaluation. To date, the requirement for radiopacity has limited the construction of devices to certain materials which in turn has limited the features attainable for particular applications and the available insertion techniques.